Question

Which of the following is desirable in diesel and kerosene but is undesirable in gasoline?
A.Paraffins
B.Mercaptans
C.Aromatics
D.Naphthenic acid

Answer 1

Hint: Aromatics in gasoline have high octane numbers. Fuels with high octane numbers are used in high performance gasoline engines as more compression is required for fuel ignition. But aromatic in gasoline have carcinogenic effects as they are released in exhaust particulates.

Complete Step-by-Step answer:
Aromatics are desirable in diesel and kerosene but undesirable in gasoline as it is a mixture of paraffin, naphthene and olefins. Aromatics are a base component of gasoline which are derived from crude oil. These substances are manipulated by refiners as they are mainly mixtures of chemicals like benzene, toluene, and xylene so as to increase the octane rating.
Octane rating is a measure of how well the fuel actually works and its ability to resist knocking when ignited in a mixture with air in the cylinder of an internal engine of combustion. Aromatics in gasoline have high octane numbers as they are used in high performance gasoline engines. However, aromatics may worsen engine cleanliness and also increase engine deposits, needed for new sophisticated engines and after-treatment devices.
Kerosene and Diesel fuel contains a large selection of hydrocarbons ranging from 10-22 carbon atoms per molecules having three major classes: alkenes, cycloalkanes, and aromatics. In them, aromatics are known to increase oxides of nitrogen emissions as they produce higher combustion temperatures.
Aromatic hydrocarbons are HC compounds containing one or more benzene-like ring structures. They are distinguished from paraffin and naphthenic which lack such structures. Aromatic hydrocarbons are denser and have poorer self-ignition qualities, and also, they produce more soot in burning.
We can note that straight run diesel fuel produced by simple distillation of crude oil is low in aromatic hydrocarbons. Catalytic cracking of residual oil is done to increase gasoline and diesel production. But more highly aromatic fuels have a greater tendency to form deposits on fuel injectors and other critical components. Such deposits then interfere with proper fuel-air mixing, by increasing particulate matter and hydrocarbon emissions.
We must know that aromatics may lead to carcinogenic compounds in exhaust gases, such as benzene and polyaromatic compounds. Polynuclear aromatic compounds, PAC were characterized in diesel and kerosene fuel as well as in their respective engine exhaust particulates. Diesel fuel contain high amounts of different PAC, a total concentration of 14,740 ppm including carbazole and dibenzothiophene, which are famous carcinogens. Kerosene fuel contain high amounts of different PAC, a total concentration of 10,930 ppm mainly of lower molecular weight such as naphthalene and its alkyl derivatives.
Thus, we can say that aromatics is desirable in diesel and kerosene but is undesirable in gasoline.
Hence, the correct option is (C).

Note: Polycyclic aromatic hydrocarbons (PAH) are also included in a high number of compounds present in the group of unregulated pollutants which are emitted from vehicles. Also, aromatic fuels have more energy density since they are denser than the non-aromatic fuels.